This invention relates to determining a direction and/or a parallelism of a beam, e.g., a charged particle beam. In particular, the invention relates to adjusting an intensity profile of a portion of an ion beam and detecting the adjusted intensity profile to determine a parallelism and/or a direction of the ion beam.
It may be important to know and control the direction and/or parallelism of a beam for the proper operation of various different types of devices and processes. As used herein, a parallelism of a beam is a measure of the extent to which the trajectories of different particles in the beam or different rays of the beam are parallel to a reference direction. Beam direction refers to the overall direction of the entire beam, not necessarily the direction of any particular particle or ray.
Ion implantation is a standard technique for introducing conductivity altering impurities into, or doping, semiconductor wafers. A typical ion implantation process uses an energetic ion beam to introduce impurities into the semiconductor wafers. As is well known, introducing the impurities at a uniform depth and dose into the wafers is important to ensure that the semiconductor devices being formed operate properly.
The depth at which impurities are implanted depends in part upon the angle of incidence of the ion beam pelative to the crystal structure of the semiconductor. Therefore, it is important to control the direction and/or parallelism of the ion beam during implantation to maintain a desired angle of incidence of the ions relative to a wafer""s crystal structure, particularly when scanning the ion beam across a wafer surface. The direction of the ion beam is the overall orientation of the beam relative to the wafer or some other reference direction or surface. The parallelism of the beam is the relative degree of collimation of the beam, or a measure of convergence or divergence of two or more portions of a stationary or scanned beam.
When an ion beam is scanned across a wafer, the angle of incidence of the ions relative to the wafer can vary from one end of the scan to the other. Since both the direction and parallelism of the ion beam affect the direction that each ion impacts the wafer, knowing and controlling the direction and parallelism of the ion beam is useful in ensuring that the doped semiconductor wafer has desired characteristics. For example, if a scanned ion beam is known to have components at opposite ends of the scan line that are not sufficiently parallel to a desired direction, portions of the implantation system, such as an angle corrector magnet, can be adjusted to make the beam components more parallel to the desired direction. Even when a fixed ion beam is utilized and the wafer is mechanically scanned, measurement and control of beam parallelism and direction are needed to ensure that the ions in the ion beam are incident on the wafer at a desired angle of incidence over the area of the ion beam.
To date, no method for measuring the direction and/or parallelism of an ion beam in situ, i.e., while an ion implantation system is configured to implant a semiconductor wafer, are known. Instead, other methods, such as testing the characteristics of a doped semiconductor wafer, have been used to measure the direction and/or parallelism of an ion beam. These methods do not allow easy and rapid measurement and control of the ion beam direction or parallelism.
The invention provides a method and apparatus for determining a direction or parallelism of a beam. The beam can be a charged particle beam, such as an ion beam, a beam of electromagnetic radiation or other beam. In one aspect of the invention, an adjusted intensity profile is created at a first position from at least a portion of the beam. The adjusted intensity profile can be created at the first position, for example, by blocking or otherwise adjusting the intensity of a portion of the beam using a beam modifier, such as a mask or other intensity adjusting element. An intensity profile is detected downstream of the first position and can correspond closely to the adjusted intensity profile. For example, if the adjusted intensity profile is or includes a circular shadow portion, i.e., a circular portion of the beam is blocked, the detected intensity profile can be or include a circular shadow portion. Based on the position of the detected intensity profile relative to the position where the adjusted intensity profile is created, a measure of the beam""s parallelism or direction can be determined. For example, if a circular portion of a beam is blocked at the position (0,0,5) where the position of the circular portion is measured in a (X,Y,Z) coordinate system, and a circular shadow is detected at the position (0,0,0), a determination can be made that the beam is parallel to the Z axis. In addition, if other portions of the beam have been determined to be parallel to the Z axis, or if other portions of the beam are assumed to be parallel to the Z axis, a determination can be made that the beam is a parallel or collimated beam.
In one aspect of the invention, a measure of the direction and/or parallelism of a beam can be made simultaneously with a determination of the uniformity of the beam. For example, a beam modifier that acts as a beam uniformity detector can be scanned across a beam, thus simultaneously providing an indication of uniformity of the beam while creating an adjusted intensity profile of a portion of the beam. An intensity profile of the beam can be detected downstream of the beam modifier and used similarly to the way discussed above to determine parallelism and/or direction of the beam. In another aspect of the invention, a determined direction or parallelism for the beam can be used to control generation of the beam so that the beam travels in a desired direction or parallelism.
The invention also provides an apparatus for determining a direction or parallelism of a beam. The apparatus may be a part of an ion implantation system and include a beam generator and a beam modifier that blocks at least a portion of an ion beam or otherwise alters an intensity profile of the ion beam. The beam modifier can be, or include, a detector, such as a detector used to determine a uniformity of the beam, or any other object that alters an intensity profile of at least a portion of the beam. The beam modifier can be fixed in place or moved to adjust the intensity profile of different portions of the beam. At least one detector downstream of the beam modifier can detect an intensity profile of at least a portion of the beam, and a controller can determine the direction of the beam and/or parallelism of the beam based on the relative positions of the beam modifier and the detector as well as the detected intensity profile. If the beam modifier is movable, the controller and optional drive mechanism can be used to control the movement of the beam modifier relative to the beam. Similarly, the controller and optional drive mechanism can move a detector relative to the beam. That is, the beam modifier can remain stationary and a detector can be moved relative to the beam and beam modifier to detect a beam intensity profile downstream of the beam modifier.